The present invention relates generally to ultrasonic imaging catheters, and more particularly, to ultrasonic transducers providing improved resolution for such catheters.
Intravascular imaging of blood vessels and surrounding tissues continues to be of great benefit in a wide range of medical fields. A particularly successful design for an intravascular imaging catheter 10 is shown in FIGS. 1A and 1B. Catheter 10 employs a rotatable imaging assembly 12 having a distal end 16 and a proximal end. An ultrasound transducer 14 is attached to distal end 16. The proximal end is operably attached to a flexible drive cable (not shown). Transducer 14 typically is elliptical in shape with a flat outer face. The transducer outer face has its major axis aligned with a longitudinal axis 20 of the imaging assembly 12. In other cases, the transducer 14 is round in shape with a flat outer face as shown in FIG. 1C.
During operation, a flexible sheath 18 is inserted into a patient with the drive cable and imaging assembly 12 disposed within sheath 18. The imaging assembly 12 typically is rotated within sheath 18 during transmission of ultrasound signals into the patient. During rotation of imaging assembly 12, transducer 14 projects ultrasound signals into a 360 degree image plane. The image plane has an in-plane or X-plane component 22 created primarily by the rotation of transducer 14. The image plane also has a cross-plane or Y-plane component 24 created primarily by the length of the major axis of transducer 14 for the transducer shown in FIG. 1B. The transducer element 14 is connected to electronics, typically maintained outside the patient's body, to produce a video image of at least a portion of the image plane by well-known techniques.
To produce images, it is desirable to have ultrasound signals transmitted by transducer 14 pass through sheath 18 and reflect off of tissue or fluids. However, a portion of the ultrasound signals transmitted by the transducer 14 typically are reflected by the sheath 18. Another portion of the ultrasound signals pass through sheath 18, but are refracted by sheath 18 during passage.
Due at least in part to the sheath effects on the ultrasound signal and to the shape of the transducer, ultrasound signals typically have a different in-plane profile than a cross-plane profile. The in-plane profile typically is narrower or tighter than the cross-plane profile. This can be seen by comparing FIG. 2A (depicting an in-plane profile 26 for a round transducer) with FIG. 2B (depicting a cross-plane profile 28 for a round transducer). Further, the in-plane profile 26 has a focal length that is shorter compared to the focal length in the cross-plane profile 28. As a result, the transducer 14 has better lateral resolution in the in-plane direction 22 than in the cross-plane direction 24.
It is desirable, therefore, to produce a tighter beam profile in the cross-plane direction so that the focal point is closer to the transducer surface. Improved cross-plane lateral resolution will result. It is further desirable to provide a more circular or symmetrical cross-section for the ultrasound signal profile, so that lateral resolution is similar for both the in-plane and cross-plane.